1. Field of the Invention
The invention relates to a piston-type compressor for gaseous media, e.g., a brake air compressor for automotive vehicles, and in particular to a compressor having capacity control means, wherein the piston-type compressor is provided with an externally arranged lubricant pump for supplying lubricant from a storage reservoir and through the lubrication circuit of the compressor, and wherein a return duct for the lubricant leads from the otherwise enclosed crankcase of the compressor back to the storage reservoir.
2. The Prior Art
Piston-type compressors of the noted type are used in particular as brake air compressors for automotive vehicles. The compressor is driven by the engine of the automotive vehicle and is supplied with lubricant by the lubricant pump of the engine. Since brake air is required in an irregular manner, the compressor delivers it into a pressure receiver. When the receiver has been charged to its full pressure, delivery of the compressor is interrupted either by means for capacity control, e.g., a simple idling control, or by a pressure regulator which will simply vent any excess air delivered by the compressor into the atmosphere. Use is also made of regulating means holding the compressor inlet valve in its opened position, so that the aspirated medium will be forced back again into the suction line. With known piston-type compressors, an undesirable accumulation of lubricant will occur within the cylinder bore of the compressor, particularly during steady-state operation and also when idling. Such excess lubricant will cause excess oiling of the delivered gaseous medium. In brake compressors, such excess lubricant is vented into the atmosphere with the brake air, and during steady-state operation also with the excess air. Since brake air compressors are, as a rule, connected to the lubrication circuit of the automotive engines, these engines will show an increase lubricating oil consumption. Such excessively oiled air, furthermore, will cause increased environmental pollution, particularly on public thoroughfares such as, e.g., pull-offs at bus stops, where such excessively oiled air is vented. This problem cannot be solved by a decreased lubricant supply to the compressor, since this would engender the risk of overheating. Returning the excess air with its entrained oil to the intake manifold of the vehicle's engine is not only expensive but is also objected to by the engine manufacturers since, apart from the excessively oiled air, coked oil and other contaminants could also enter the combustion chamber.